A boring tool is used for the rough machining or finish machining of a hole provided in a workpiece. For example, a line boring bar, which is a type of boring tool, is used when machining a crankshaft hole of a cylinder block of an internal combustion engine. The machining of the crankshaft hole is performed by inserting the line boring bar into the crankshaft hole of the cylinder block, and reciprocating the line boring bar in the axial direction while rotating the line boring bar.
Since the line boring bar makes such a motion, it is required to have both of abrasion resistance and high stiffness. Thus, a tool made of steel has so far been used as the line boring bar. However, a further improvement in the machining accuracy of the tool is desired, and deformation and vibration due to the mass of the boring tool itself are not negligible any longer. Thus, further weight reduction is under consideration. For example, Patent Document 1 discloses a high-stiffness composite material bar. The high-stiffness composite material bar is composed of a cylindrical metal bar produced from a material with high static stiffness; a cylindrical composite material bar connected to one end of the metal bar and produced from a composite material with high specific stiffness or rigidity; and a connecting member surrounding the outer peripheries of the metal bar and the composite material bar.